Adaptive filters operate on an incoming signal to generate an impulse response characteristic in accordance with a prescribed algorithm. Existing adaptive filters, for example, those used as echo cancelers are able to model an impulse response of a limited interval, for example, 16 milliseconds. In some applications, it is desirable to model an impulse response having an interval greater than the available interval of an individual adaptive filter or echo canceler. One example, is in applications where the round trip electrical delay encountered in a telephone transmission channel is greater than the impulse response interval of an individual echo canceler. Another example, is in applications for addressing acoustic delays where the impulse response needed can be extremely long.
One solution to the long electrical delay problem is to use a "truly" cascadable adaptive echo canceler. In such an arrangement, each canceler uses a common error signal to update the impulse response estimate being generated. In order to do this, the individual adaptive cancelers must be arranged circuit wise to facilitate inputting the common error signal to the impulse response updating circuitry. Such a truly cascadable echo canceler including an adaptive filter is manufactured by Western Electric Company. However, most presently available echo cancelers do not have this capability. Moreover, only a limited number of these truly cascadable echo cancelers can be cascaded and, therefore, their use is essentially limited to addressing the long electrical delays and not the extremely long acoustic delays.
One attempt at using available echo cancelers to obtain an extended impulse response characteristic is disclosed in copending U.S. patent application Ser. No. 464,054, filed concurrently herewith. In the disclosed arrangement, two adaptive echo cancelers are arranged in tandem and their relative positions in the tandem connection are controllably switched in order to improve the overall adaptation to a desired extended impulse response. One problem with this arrangement is that it must be determined when to switch the cancelers. Another problem is that ON-HOOK, OFF-HOOK information is needed to initiate and reset the canceler adaptation process. This type of information is not usually available for echo canceler applications. Moreover, because of the need to determine when to switch the cancelers, this arrangement is not readily expanded beyond the two adaptive echo canceler applications. Thus, although this tandem arrangement may be satisfactory for some applications, it is not for others, for example, the canceling of an acoustic echo and where ON-HOOK, OFF-HOOK information is not readily available.